Tap



Feb. 16, 1932. N. TRBoJr-:vxcH 1,845,063

TAP

Filed July 11. 195.7 -2 sheets-sheet 1 Patented Feb. 16, 1932 .UNITED STATES NnrortnA frmsoanvrcn, or HIGHLAND PARK, MICHIGAN TAP Application led July 11, 1927. rSerial No. 204,951. y

The invention relates to an improvement in taps such as are used for cutting screw threads in circular vholes.

Heretofore taps have been made in two principal forms or types, viz. those having their cutting teeth backed off7 or relieved, and those having unrelieved or cylindrical teeth. and also undesirable characteristics. rlhe re- 10 lievedy tap cuts freely and produces a good finish, but it is likely to damage the screw threads when it is `withdrawn from the threaded hole and has the further disadvan-y tage that the diameter becomes smaller after .each sharpening. rlhe unrelieved tap may be easily withdrawn fromthe threaded hole and also holds its size after sharpening, but it does not cut as freely and rapidly as a relieved tap and, asa rule, it does not produce as good a finish.

l have conceived the idea of constructing a tap in which all teeth that do the actual cutting` will be relieved 'so that they may cut freely and yet the tap can be withdrawn from the hole without damaging the threads and it also can be resharpened without losing its size. As it will be hereinafter fully explained, the new method of making such taps is rather simple, accurate and economical and 30 may be performed in standard machine tools. The method is also applicable to making of hobs of the tangential feed type such as are used for hobbing of worm gears.

ln the drawings; v

355 Figure 1 is a side view of my improved tap in which the unrelieved and theV relieved portions of cutting threads are indicated by the letters a and respectively. Y

Figure 2 is the cross section in the plane 40" 2-,2 of the tap shown in Figure 1.

Figures 3, l and 5 are diagrams explana tory to the theory of the process.

Figure 6 is a diagrammatical viewof a.

Each type possesses certain desirable threaded portions 12. Each threaded portion 12 has a series of thread segments ory teeth B, B', B2 etc. extending circumferentially from he cutting faces 15 formed by one flute tothe rear faces formed by the next flute. According to my invention each thread segment orl tooth from B to C is relieved or backed off for a portion of its circumferential width, the relieved portions in adjacent teeth being progressivley decreased in width from B to C. As shown in `Figure 1 the helix 14 forming an angle with thecutting face 15 divides the threaded portion 12 into two substantially triangular areas a and b, the area Z) representing the relieved portions of the teeth and the area a the unrelieved or cylindrical portions.` It ywill be seen from the drawings that'the portion of the thread extending from A to B consists of roughing teeth only, allof which are relieved and all of which gradually increase in thickness orL width from A to B. The first finishing tooth is at B and is of a full width and fully ref lieved, that is it has no cylindrical land. The next iinishing tooth B has a narrow cylindrical land adjacent the cutting edge and the following tooth B2 has aland wider than that of B. Considering the cutting teeth consecutively from B to C it willbe noted that all possess cylindrical lands of an ever increasing width until at the point C they are cylindrical for practically the entire width.

- The practical consenquence is thiszlVhen the tap is fed into work from A to B the teeth allcut freely because they are relieved. From B to C the teeth ,dok very little cutting because the thread is alreadyof full size at B. After the work has been tapped through, the rotation of the tap is reversed in order vto back out. Now, the threads at C will beA the leading ones, and being substantially cylindrical, lwill `back out without kmutil'ating the thread, and in backing out will also guide the following roughing teeth from B to A 'so that they also rwill back out through the thread without mutilating the same.

In Figure 3 a development of a helical cutting thread of theimproved tap is dia-grammatically shown.,l Altogether eight teeth from Zl to ZS inclusivey are shown for the purl pose of a better illustration of the principle although a complete tap thread in reality may contain a hundred or more cutting teeth.

The method of manufacturing the new taps will now be explained. Figure 6 shows diagrammatically the essential parts of a conventional back-off machine. The work l1 r0- tates in the direction of the arrow 16 with a constant angularvelocity el. The object is to relieve the cutting teeth 15 so that their cutting faces adjacent to the flutes 13 will stand out prominently relative to the rear portions of said teeth. A cam 17 of the Archimedean spiral type is rotated in the direction of the arrow 18 with a uniform velocity c2. The thread tool 19 is firmly held in the post 20 and said post is slidable in the ways 21 of the machine bed 22. rlhe sliding member 23 also contains the cam follower 24, and presses against the cam 17 with a considerable force derived from the coil spring 25, said spring being anchored at its one end to the bed 22 and at its other end to the slide 23.

The conventional process of backing off taps or any other cutters consists of rotating the work 11 and the cam 18 in such a ratio that the cam will make one full turn while the work rotates from one flute 13 to the next. If the work is helical as is always the case with taps, the work 1l is also given a translation along its axis to generate the desired lead of helix. The highest point D of the cam corresponds to the lowest point D of the Archimedean spiral se ment 26 in the work 11 and the lowest point of the cam corresponds to the highest point E in the work.

My improvement consists in so changing the ratio of the timing of the respective rotations of the cam and the work that the spacings of the jumps of the cam, 15+ A in Figure 3 will be slightly longer than the spacings t of the consecutive flutes, although both spacings are strictly uniform during the relieving operation.

The effect may be studied in Figure 3. Suppose that the tap 11 has been chased down to the full size of thread and that the tooth Z3 is the first finishing or full size tooth. rl`he Archimedcan spiral 2G in its downward Grade strikes the tooth Z3 at the point F and hacks it oflt' along the line FG and also along the sides of the thread. The back-off tool will now rise and then again descend until at the point F of tooth Z,l it will be at the same distance from the axis of the tap as it was at the point F. Thus, a cylindrical land FH is formed in the front of the tooth Z4. If the spacing of the cam jumps is equal to H- A and that of flutes to t, it is seen that the width FH=A. The width of land at the next tooth Z5 is equal to 2A, at Z6 to 3A, etc.

It is now necessary to calculate the increments to suit the purposes previously discussed. If there be n threads in the selected lengthvof tap BC, Figure 1 and N flutes, it

A :Nn (l) The path of the back-off tool relative to all cutting teeth may be readily followed in Figure 4 which represents the pitch cylinder of the tap as developed in a plane. The abscissa D11- is the circumference of pitch circle and upon the ordinate are measured off the disstances L corresponding to the lead of tap thread. The cutting faces 15 are now a series of parallel vertical lines because they are what is termed straight flutes. If the flutes were helical they would be inclined in development at an angle relative to the axis Dvr, but that Would not change the principle of this construction which is equally applicable to both straight and helical flutes.

As the back-off tool jumps from Z2 to Z3, Z5, etc. it always keeps on gaining upon the consecutive cutting teeth and so produces the required cylindrical lands of an ever increasing width. The increase is in the manner of an arithmetic series and may be obtained by constant velocity gearing. It is only necessary to gear up the work 11 and the cam 17 Figure 6 in the roper ratio. If a conventional tap were acked off in that machine the ratio of the cam velocity ful to that of the work velocity v2 would be four because the tap has four straight flutes. tem the ratio would be In my sysi. e. slightly less than four to produce the rehelix 14 Figure 4 i. e. the locus of the begin- 115 ning points of the relieved tooth portions, and not the helix 15 forming the cutting edges as according to standard practice.

When the ta 11, Figure 1 is hardened it may be groun cylindrically in the portion a of the threads. It is seen that this kind of tap grindin is very economical because first it is cylin rical and second it is necessary to grind only a small portion of the screw thread.

When atap of this kind gets dull, its sharpness is restored by grinding over the faces 15, Figure 1. By inspecting said figure it is seen that by removing a film of metal parallel to the plane 15, an equal amount is subtracted from the Width of each cyindrical land, but that otherwise the conditions do not change in the least, i. e. the tap Will hold its form and size after sharpening, and the lands Will be progressively broadening in the same manner as before the sharpening.

What I claim as my invention is l. A thread cutting tool having a threaded portion interrupted by longitudinally eX- tending flutes forming cutting teeth, the teeth at the entrance end of the tool being relieved for the entire circumferential length thereof and the successive teeth toward the rear end of the tool having progressively increasing circumferential length of the full thread diameter.

2. A tap in Which the cutting teeth are of the full thread diameter for progressively increasing circumferential lengths from the entrance end of the tap to the opposite end thereof, the remaining portions of said cutting teeth being relieved.

3. A tap comprising a threaded portion interrupted by longitudinally extending flutes forming cutting teeth, the teeth having a p0rtion of their circumferential lengths of the full diameter and a portion relieved to a lesser diameter, the successive teeth from the rear end of the tap toward the entrance end thereof being relieved for progressively increasing circumferential distances.

4. In a thread cutting tap a plurality of cutting teeth helically aligned and provided with cylindrical lands of progressive Widths and adjacent to the cutting edges, the Width of said lands increasing in an arithmetical series from t-he entering towards the rear end of said tap.

5. In a thread cutting tap a plurality of helically aligned cutting teeth, each tooth consisting of a relieved and an unrelieved portion so arranged that the Widths of the relieved portions diminish, and the Widths of the unrelieved portions increase, from the entering toward the rear end of said tap.

In testimony whereof I affix my signature.

NIKOLA TRBOJEVICH. 

